K. H. Chae

ROOM TEMPERATURE FERROMAGNETISM IN PURE AND Cu DOPED ZnO NANORODS: ROLE OF COPPER OR DEFECTS

We report structural, magnetic and electronic structure studies of pure and Cu doped ZnO nanorods with the aim to understand the origin of ferromagnetism. A structural study indicates that all the samples exhibit single phase nature and rules out the formation of secondary phase. NEXAFS measurements reveal that Cu ions exist in Cu2+ state. Magnetic hysteresis loop measurements reflect that the pure and Cu doped ZnO nanorods exhibit room temperature ferromagnetism. The increase in the intensity of green emission in photoluminescence study indicates that defects density increases with Cu doping.

NEXAFS studies of La0.8Bi0.2Fe1−xMnxO3(0.0 ⩽ x ⩽ 0.4) multiferroic system using x-ray absorption spectroscopy

We report the electronic structure studies of well-characterized La0.8Bi0.2Fe1−xMnxO3 (LBFM, 0.0 ≤ x ≤ 0.4) multiferroic samples investigated by soft x-ray absorption spectroscopy at O K-, Mn L3,2-, Mn K-, Fe L3,2- and La M4,5-edges along with the spectra of reference compounds. The investigations are performed to find out the chemical states and crystal field symmetry of ions present in the LBFM multiferroic samples after the substitution of Mn ions at Fe sites. Studies reveal a slight shift in the peak positions of O K-edge spectra along with growth in intensities of low-energy features. An analysis of the Mn L3,2- and K-edge, in addition to the comparison with the spectra of reference compounds, indicates that the Mn ions are present in mixed valence states. It is found that the Mn ions are mainly in +3 states along with a small contribution of +2 and +4 ones. The study of Fe L3,2-edge confirms the trivalent state of Fe ion. M4,5-edges of La corroborate the +3 state of La in the system. Atomic multiplet calculations are also performed on Fe L3,2-, Mn L3,2- and La M4,5-edges to further verify the valency, symmetry and the crystal field splitting.

Role of defects in BiFeO3 multiferroic films and their local electronic structure by x-ray absorption spectroscopy

Present study reports the role of defects in the electrical transport in BiFeO3 (BFO) multiferroic films and its local electronic structure investigated by near-edge X-ray absorption fine structure. Defects created by high energy 200 MeV Ag+15 ion irradiation with a fluence of ∼5 × 1011 ions/cm2 results in the increase in structural strain and reduction in the mobility of charge carriers and enhancement in resistive (I-V) and polarization (P-E) switching behaviour. At higher fluence of ∼5 × 1012 ions/cm2, there is a release in the structural strain due to local annealing effect, resulting in an increase in the mobility of charge carriers, which are released from oxygen vacancies and hence suppression in resistive and polarization switching. Near-edge X-ray absorption fine structure studies at Fe L3, 2- and O K-edges show a significant change in the spectral features suggesting the modifications in the local electronic structure responsible for changes in the intrinsic magnetic moment and electrical transp...

Ion beam synthesis of Ni nanoparticles embedded in quartz

Fabrication and fluence dependent growth of Ni nanoparticles in quartz matrix using a low energy ion implantation followed by thermal annealing are reported. 100keV Ni ions are implanted (at room temperature) in quartz at different ion fluences using a low energy ion beam facility. The samples are postannealed at 600°C and then characterized using atomic force microscopy, magnetic force microscopy, field cooled and zero field cooled magnetization measurements, x-ray absorption spectroscopy, and UV-visible spectroscopy. Nanoclusters of Ni of nearly uniform size are synthesized at an ion fluence of 5×1016ions∕cm2. The size of these nanoclusters could be varied by varying ion fluence. Formation of NiO nanoclusters is observed for ion fluence of 2×1017ions∕cm2.

Electronic structure and magnetic properties of the Ni0.2Cd0.3Fe(2.5-x)A1(x)O4 (0 < or = x < or = 0.4) ferrite nanoparticles.

The structural, magnetic, and electronic structural properties of Ni0.2Cd0.3Fe(2.5-x)Al(x)O4 ferrite nanoparticles were studied via X-ray diffraction (XRD), transmission electron microscopy (TEM), DC magnetization, and near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) measurements. Nanoparticles of Ni0.2Cd0.3Fe(2.5x)Al(x)O4 (0 < or = x < or = 0.4) ferrite were synthesized using the sol-gel method. The XRD and TEM measurements showed that all the samples had a single-phase nature with a cubic structure, and had nanocrystalline behavior. From the XRD and TEM analysis, it was found that the particle size increases with Al doping. The DC magnetization measurements revealed that the blocking temperature increases with increased Al doping. It was observed that the magnetic moment decreases with Al doping, which may be due to the dilution of the sublattice by the doping of the Al ions. The NEXAFS measurements performed at room temperature indicated that Fe exists in a mixed-valence state.

Tailoring of absorption edge by thermal annealing in tin oxide thin films

Tin oxide (SnO2) thin films were deposited by radio-frequency (RF) magnetron sputtering on silicon and glass substrates in different oxygen-to-argon gas-flow ratio (O2-to-Ar = 0%, 10%, 50%). All films were deposited at room temperature and fixed working pressures, 10 mTorr. The X-ray diffraction (XRD) measurement suggests that all films were crystalline in nature except film deposited in argon environment. Thin films were annealed in air at 200 °C, 400 °C and 600 °C for two hours. All films were highly transparent except the film deposited only in the argon environment. It was also observed that transparency was improved with annealing due to decrease in oxygen vacancies. Atomic force microscopy (AFM), results showed that the surface of all the films were highly flat and smooth. Blue shift was observed in the absorption edge with annealing temperature. It was also observed that there was not big change in the absorption edge with annealing for films deposited in 10% and 50% oxygen-to-argon gas-flow ratio.

MAGNETIC AND ELECTRONIC STRUCTURE STUDIES OF Mn1+xFe2-2xTixO4 (0 ≤ x ≤ 0.5) FERRITES

A series of polycrystalline bulk samples of Mn1+xFe2-2xTixO4 (0 ≤ x ≤ 0.5) have been prepared by using a conventional solid-state reaction method. The effect of Ti4+ ions substitution on magnetic and electronic structure of MnFe2O4 has been studied using isothermal d.c. magnetization and near edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements. The magnetization studies infer that all samples have ferrimagnetic ordering at room temperature. The magneton number (nB) calculated using the magnetic hysteresis loop has been found to decrease linearly with the Ti substitution, which indicates that the system follows the Neels two sub-lattice model. The decrease in the value of nB may be due to the dilution of the sub-lattice by Ti ions. The NEXAFS measurements performed at Fe/Ti L3, 2 edges indicate that Fe ions are in mixed valence state whereas Ti ions remain unchanged at 4+ state.

Design and fabrication of a micromechanical inverter

A micromechanical switch, which can be used as a logic gate, is described in this paper. This switch consists of fixed input electrodes, output electrode, VCC/GND pad, and movable electrode suspended by crab-leg flexures. For mechanical switching of an electrical signal, an actuator driven by electrostatic force was used. Provided that a movable electrode is connected to VCC and a low input voltage or ground signal is applied to the fixed input electrodes and the movable electrode. The proposed micromechanical switch was fabricated by surface micromachining technology with 2 micrometers -thick poly-Si and the measured threshold voltage for ON/OFF switching was 23V.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Structural and magnetic properties of Fe doped CeO2 thin films

Pure and Ce0.95Fe0.05O2 thin films have been fabricated on LaAlO3 (100) substrate using pulsed laser deposition techniques at 200mTorr oxygen partial pressure and then characterized by various techniques viz. X-ray diffraction, atomic force microscopy, NEXAFS, and dc- magnetization measurements to explore the structural and magnetic properties of pure and Fe doped CeO2 thin films. The XRD patterns indicate that pure and Ce0.95Fe0.05O2 films have single phase polycrystalline behavior. The surface roughness measured using AFM microscopy found to decrease with Fe doping and revealed that both the films are nanocrystalline. The electronic structure measured at Fe L3,2 edge infer that Fe-ions in mixed valence states (Fe3+ and Fe2+) in Fe doped CeO2film. DC-magnetization measurements showed that pure and Fe doped CeO2 films have ferromagnetic ordering at room temperature.Pure and Ce0.95Fe0.05O2 thin films have been fabricated on LaAlO3 (100) substrate using pulsed laser deposition techniques at 200mTorr oxygen partial pressure and then characterized by various techniques viz. X-ray diffraction, atomic force microscopy, NEXAFS, and dc- magnetization measurements to explore the structural and magnetic properties of pure and Fe doped CeO2 thin films. The XRD patterns indicate that pure and Ce0.95Fe0.05O2 films have single phase polycrystalline behavior. The surface roughness measured using AFM microscopy found to decrease with Fe doping and revealed that both the films are nanocrystalline. The electronic structure measured at Fe L3,2 edge infer that Fe-ions in mixed valence states (Fe3+ and Fe2+) in Fe doped CeO2film. DC-magnetization measurements showed that pure and Fe doped CeO2 films have ferromagnetic ordering at room temperature.

Near-edge X-ray absorption fine structure spectroscopy and structural properties of Ni-doped CeO2 nanoparticles

ABSTRACT Ni-doped CeO2 nanoparticles were prepared by using the co-precipitation method. The prepared nanoparticles were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The XRD results infer that Ni-doped CeO2 nanoparticles have single phase nature similar to that of pure CeO2 nanoparticles. We have calculated lattice parameters using Powder-X software, particle size using Scherer’s formula and strain using the Williamson-Hall method for all the synthesized samples. We have observed a systematic decrease in the lattice parameters, particle size and strain with an increase in Ni doping in CeO2. The FE-SEM micrographs also confirm that Ni-doped CeO2 have nanocrystalline behavior and particles are spherical shaped. From the Raman spectra, it is observed that the intensity of classical CeO2 vibration modes first increases then decreases with Ni doping. The NEXAFS spectra measured at Ce M4,5 and Ni L3,2 edges clearly indicate that Ce ions are in the +4 valence state and Ni ions are in the +2 valence state.